1. Rangers, if not using a shield, get an extra attack per round. Furthermore, paladins and rangers get access to spells, including 6th level spells at level 29. (Yes, this means they get access to Heal eventually, but probably not until Heart of Fury mode.) Also, note that Paladins do get Turn Undead, and undead creatures are actually quite common. (For example, the first major multi-floor dungeon in the game is filled with skeletal undead.)

2. Grandmastery is actually quite powerful; you get yet another half attack (in addition to what Specialization gives you). Note that High Mastery isn't worth it; either stop at Mastery (+3 to hit and damage) or go all the way.

3. I don't know the details, but you will likely want a diverse selection of them. Skeletal undead, for example, tend to have resistance to slash and pierce damage, but not blunt damage. (The game tends to make more use of the ability to give creatures resistance to specific types of physical damage more than the Baldur's Gate games.)

4. I think the specializations are a bit more balanced than in BG2; everyone has you giving up some nice spells. One thing to note: A mage without Conjuration is completely unable to use 9th level spells. (On Heart of Fury mode you *will* want summon spells, which means you will need Conjuration or Illusion.) Also, keep in mind that scrolls are scarce in this game, so you may (especially if using a single-class specialist) end up in situations where you have spell slots for a new level of spells, but no spells to spend them on. Also, parties focused heavily on arcane magic will have problems with the limited amount of scrolls available.

Also, note that Bards get multiple songs, and some of them are very useful, so I recommend taking one along.

All a matter of preference. If sheer power is your top concern, Fighter delivers. It doesn't suffer the problem that it has in BG2, and gives you the extra attack per round as promised. For the most part, you'll be fine. The game has a reasonable distribution of drops and there's at least something decent for every weapon category if memory serves. More important is avoiding too many characters specialized in the same weapon type, since they end up competing for item drops. The IWD series is incredibly stingy with scrolls, so I actually recommend generalist. I normally hate the tight spell slot restrictions of a generalist mage, but when you're constantly crossing your fingers that a decent scroll will finally drop as treasure there is nothing more heartbreaking than discovering it's of your opposition school.

If you are going to specialize, just make sure you don't take the path of the Invoker. It is literally inferior to the Diviner in every way. Everything is reasonable and a matter of preference.

It's also worth noting that, unlike some D&D cRPGs, you don't get to pick spells at level ups, either. (Why the game doesn't let you do that I have no idea, especially since both Pool of Radiance and Temple of Elemental Evil do.)

1. All three classes have their merits:

Paladins are the best defensive tanks because their "Protection from Evil" special ability stacks with almost everything and last very long, giving them another +2 to AC and +2 to saves on top of their natural +2 to saves.
Since you can only dump int their important physical stats will be worse than the other warriors'.

Rangers get an extra melee attack per round without a shield and therefore do more damage from the beginning. If you want to switch from melee to bow comfortably you won't want to use a shield anyway.
They also get access to some druid spells which is nice if you don't have a pure druid.

Fighters level faster and do by far the most damage once they get grandmastery at level 9. They'll only be able to grandmaster one weapon type and master a second.

2. It's very powerful. Level 9-12 warriors get 2 attacks per round if specialized and 3 with grandmastery. Add the extra bonus to hit and damage and you're almost twice as effective. Unlike stated in the manual attacks per round are capped at 10 hasted, not at 5.

3. Which weapon types are best depends on where in the game you are and also on some random weapon drops. Spread the weapon proficiencies among your party member so you can take advantage of the different weapon types you find. Large swords are the most common, followed by axes (proficiency is also used for throwing axes).
Clubs, quarter staves and halberds are quite rare. For druids large sword specialization gives bonuses to the polar bear shapeshift.

4. I prefer Illusionist, besides having the least important restricted spell schools they also get better rolls because of granted 16 dex.

5. As an additional party member I recommend bard, their songs let the other characters shine. The level 11 song heals the party, allowing the cleric and druid memorizing more non-healing spells. He can also learn the spells forbidden to the specialist mage and cast spells at higher caster level (longer lasting haste, ...).1

I also recommend a druid, their spells and special abilities are quite useful. Pure class, multiclass or dualclass fighter 7(for mastery in scimitars and slings)>druid all work.